There is an increasing tendency for larger number of pixels on image sensors used in devices such as digital still cameras, mobile phone cameras and optical computer pointing devices, for example. Consequently, sensors have either increased in size or pixels have been manufactured smaller, or often both. The use of finer geometry technologies increases the chance of defects occurring and reduces yield. Typically, the defectivity of an image sensor is proportional to the area.
Defects are caused during manufacture and are usually caused by dust particles obstructing the photolithography process. Resulting defects may be open circuit or short circuit connections. If a defect occurs within a pixel during manufacture, a defective pixel is usually the result. The defective pixel can be either ignored by the user or, if the defective pixel can be identified, corrected for by interpolating between neighboring pixels.
If the defect occurs on a connection that is common to either a row or column of the pixel array, then a series of pixels in the row or column may be defective, rather than a single pixel. In some Cases, the entire row or column can be defective. Defects that disrupt the operation of more than one pixel are far more noticeable to a user and much harder to compensate for.
Typically, as shown in FIG. 1, a pixel array 100 has a matrix of pixels 110. Each column of pixels 110 in the pixel array 100 is connected by a common column bitline 120. Each row of pixels 110 is connected by a common row select 130. When the row select 130 is activated (set to “high”) by row drivers 140, the pixels 110 in that row are enabled for readout and the values of the pixels 110 are read out in parallel on to the column bitlines 120 to readout amplifiers 150.
Redundancy on the row select 130 can be generated by adding additional row drivers 160 on the right of the array as shown in FIG. 2. The row select 130 can then be activated by both the left row drivers 140 and the right row drivers 160, mitigating open circuit defects. As the drivers are relatively small and as the drivers on both sides generate the same signal, there is no need to determine if or where a defect exists, it is merely sufficient to drive the signal.
Short-circuit defects are mitigated during design by increasing the spacing of adjacent metal tracks. The problem is not as simple for the column bitline, as this is the output of the pixel, which must be received or detected. A fault in the bitline will produce an error in all the pixels that are further away from the detection circuit, usually an amplifier, than the defect. Providing additional receivers at the top of the device is impractical as it will significantly increase the area of the sensor and it is also impractical to determine which is the correct signal and which is incorrect.
The traditional method to avoid open circuit bitline connections on large-area sensors, is to use wider traces as these are more immune to defects, however a wider metal conductor prevents light from reaching the sensor and degrades pixel performance. U.S. Pat. No. 6,741,754 Hamilton, “Correcting for defects in a digital image taken by an image sensor caused by pre-existing defects in two pixels in adjacent columns of an image sensor”, discloses a method for correcting for defects in a digital image taken by an image sensor when there are pre-existing defects in two pixels in adjacent columns of the image sensor which causes two adjacent lines of pixels in the digital image to have corrupted data.
U.S. Pat. No. 5,436,659, “Method and apparatus for determining defective pixel location”, attempts to use digital timing techniques to identify defective pixels and store their locations for correction by an appropriate technique, such as substituting a neighboring pixel value. U.S. Pat. No. 5,291,293, “Electronic imaging device with defect correction”, utilizes redundant sensor elements for defect compensation by using a plurality of arrays and pixels in one sensor used to correct info on the other sensor.